For an optical element to be used for digital copying or digital cameras, there is a molded lens.
The molded lens is superior in the fabricability of an aspherical lens to a polished glass lens and is low in cost, but has an unstable side that non-uniformity is likely to occur in the refractive index distribution inside the lens, depending on the molding conditions. The non-uniformity of the refractive index inside the lens greatly affects the optical characteristics of the lens, which may cause degradation in image forming performance.
Because of this, to stabilize the quality of molded lenses, a distribution of refractive indices needs to be measured with high accuracy.
In recent years, a GRIN (GRadient-INdex) lens in which a refractive index gradient is intentionally provided in the lens has begun to be used in an optical communication field, etc. Since the GRIN lens can separate a shape and refractive index, the expansion of the application range thereof such as imaging systems is expected.
The GRIN lens is designed such that a refractive index distribution is intentionally provided to a glass material. Thus, for stable mass production, a distribution of refractive indices needs to be measured with high accuracy.
For conventional art for measuring a refractive index to handle such issues, there is a method in which a refractive index is determined by measuring an angle of deviation by a minimum deviation method, etc., or a method in which a test object is immersed in a solution whose refractive index is known, and observed and a refractive index of the test object is indirectly measured from the refractive index of the solution, or a method (Mach-Zehnder interferometer) in which interference fringes are observed which occur in plane-wave test light with plane-wave reference light and the plane-wave test light being transmitted through a state in which a test object is immersed in a matching liquid whose refractive index is known and substantially equal to that of the test object.
FIG. 6 is a view showing a refractive index measuring apparatus using a conventional Mach-Zehnder interferometer.
In FIG. 6, light emitted from a laser light source 1 is converted into parallel light by a condenser lens 2. Thereafter, the light is branched by a half mirror 3 into reference light 4 and test light 5. The test light is reflected by a mirror 6 and thereafter transmitted through a liquid immersion cell 8 filled with a matching liquid whose refractive index is substantially equal to that of a test object 7 and the test object 7 immersed therein, and thereby causes an optical path difference according to a refractive index distribution of the test object 7. Then, the test light 5 reaches a half mirror 9 and thereafter enters an imaging device 11 through an image forming lens 10.
On the other hand, the reference light 4 is transmitted through a compensating plate 12, reflected by a mirror 13, reaches the half mirror 9, and thereafter, enters the imaging device 11 through the image forming lens 10. The compensating plate 12 is to make an optical path length from the test light 5 to the imaging device 11 and an optical path length from the reference light 4 to the imaging device 11 substantially equal to each other.
In the imaging device 11, the test light 5 and the reference light 4 are combined and interference fringes 14 occur according to an optical path difference therebetween. In the interference fringe observation, by the number of fringes at a location where the interference fringes 14 occur, an optical thickness of the test object 7 in that part can be computed, from which a difference in the refractive index of the matching liquid is computed, whereby a refractive index of the test object 7 can be determined.
For the method using an interferometer using a matching liquid, there are one using a bath that increases the temperature uniformity of a matching liquid, one that achieves an increase in accuracy by introducing a phase shift method, and furthermore, one that enables to measure a three-dimensional distribution by using a CT scan (see, for example, Patent Document 1).